This invention relates to the manufacture of glass fibres for optical communication. In particular it relates to the vapour-phase deposition of core and cladding material on a substrate and to the subsequent stage in the procedure in which a solid preform of a fibre is produced.
Vapour phase techniques involve the oxidation of a gaseous silicon compound such as SiCl.sub.4 to deposit a layer of silicon dioxide on a substrate which, in this invention, comprises the inner surface of a tube. Various dopant materials in a vapour phase are added in a controlled manner to deposit layers which differ in refractive index and in particular a core of a material is formed that has a greater refractive index than the cladding. The substrate is usually pure silica and forms the outer part of the cladding when a fibre is subsequently drawn from the solid preform which comprises the collapsed substrate tube.
TiO.sub.2, GeO.sub.2, P.sub.2 O.sub.5, and Al.sub.2 O.sub.3 are all dopants which have been used in the deposited core layers to increase the refractive index of the glass in that region. By varying the concentration of the dopants in the vapour a gradual refractive index change from the centre to the periphery of the preform can be obtained or alternatively a discrete step in refractive index between the core and cladding regions can be introduced. Both types of profiles are used for multimode fibres. In monomode fibres a step index profile is required in which the core radius is small and in which the difference between the refractive indices of the core and cladding is small compared with a multimode fibre.
Intrinsic losses in SiO.sub.2 - GeO.sub.2 fibres show a marked variation with respect to the wavelength of the transmitted radiation, and a window in the region 1.1 to 1.7 .mu.m is generally recognised as the optimum wavelength range. Thus, in a fibre with a GeO.sub.2 doped core a pure SiO.sub.2 cladding would seem to be the best choice for a ultra-low-loss (&lt;1 dB/km) fibre, since it does not introduce any additional absorbtion mechanisms. This is particularly true in the case of a monomode fibre where 30% of the power in a fibre having a normalised frequency of 2 travels in the cladding.
However, a very high temperature is required for the chemical vapour deposition and sintering of pure silica. Furthermore, heating to this temperature tends to produce distortion in the silica substrate tube. A small amount of P.sub.2 O.sub.5 added to the deposited silica cladding layers considerably reduces the deposition and sintering temperature, but leads to an increase in the refractive index of the deposited part of the cladding. This higher refractive index region forms part of an undesirable second wave guide with the silica substrate tube acting alone as the cladding. It has previously been proposed, for example, in Electronics Letters 1979 15 pp 411-413, to add a small amount of a refractive index--reducing dopant to the deposited cladding layers to compensate for the effect of the P.sub.2 O.sub.5 doping. For the additional dopant, fluorine has been proposed. The present invention in a first aspect relates to those fibres having such a compensated or nearly compensated deposited cladding layer based on silicon and including both refractive-index-increasing and refractive-index-decreasing dopants.
Such fibres, whilst alleviating the problems of high deposition temperature and of the formation of a secondary waveguide, suffer from the disadvantage of further absorbtion losses in the transmission spectrum, generally resulting from the vibrational absorbtion of bonds between the dopant material and water. In the case of P.sub.2 O.sub.5 a loss at.about.1.6 .mu.m occurs which is the first overtone of the P-OH vibration at 3.05 .mu.m; and possibly also there is a loss at wavelengths greater than 1.5 .mu.m due to the tail of the fundamental P-O vibration at 8.1 .mu.m.